Two things make teeth break: force and brittleness. As adults we have thirty two teeth. We have that many so that the tremendous forces of chewing can be distributed evenly. The average chewing force is about 162 pounds per square inch and teeth can easily withstand many times that in normal function which is up and down chewing. If we exert these forces side to side, as in during tooth grinding, that’s a whole other story. If there are large fillings in a tooth, side to side motion becomes more destructive. Combine that scenario with fewer than normal teeth in your mouth and the odds of breaking go up.
Teeth that have had root canal or old teeth which are not as vital as they once were are very brittle. Great care is taken while restoring these teeth to protect them from being over stressed by force. Many times a protective device, such as a night-guard is recommended to shield them from grinding forces that may occur during sleep. Failure to protect them usually ends in fracture.